On July 25, 1866, the US Congress established the grade of "General of the Army" for Ulysses S. Grant, and later appointed William T. Sherman (in 1869) and then Philip H. Sheridan (in 1888, just weeks before he died) to the rank. In 1919, John J. Pershing was named "General of the Armies of the United States", and held the rank until he died, in 1948. In all of these cases, the generals wore four stars, except between 1872 and 1888, when Sherman and Sheridan wore two stars with the arms of the United States in between.
The five-star rank was created by Public Law 482 of the 78th Congress, passed on 14 December 1944, first as a temporary rank, then made permanent 23 March 1946 by an act of the 79th Congress. This was done to have American officers with ranks equivalent to the field marshals of Great Britain, to reduce friction over who was allowed to give orders to whom. (The acts also created a comparable rank of Fleet Admiral for the Navy.)
Unsurprisingly, the Generals of the Army are all familiar names (note the careful timing of the first four, so as to establish clear seniority):
The insignia consists of five stars in a pentagonal pattern, with points touching.
The rank still exists today, although nobody has held it since General Bradley died in 1981.
In 1976, President Gerald Ford posthumously appointed George Washington as General of the Armies of the United States, and specified that he would always rank first among all officers of the Army, past and present.